National Instruments 370753C-01 Manual De Usuario

© National Instruments Corporation

For an example of frequency response of a simple system, refer to 
Example 5-2.

Given the single-input, single-output open-loop plant in Figure 5-3, where 
U(s) and Y(s) are the frequency domain input and output, respectively, you 
can examine its response characteristics and see how you can improve them 
using the frequency-response based control design functions in this chapter.

Figure 5-3.  Representation of the Open-Loop System

You can create the system directly in transfer function form:

sys = system(polynomial(-0.5), 

polynomial([0,0,-2,-10]));

and then obtain the frequency response directly:

H = freq(sys,{Fmin = 0.01,Fmax = 10,npts = 150});

freq( )

 also can be called with a predefined vector of frequency points, 

or you can specify that phase tracking be used to compute frequency points 
between the minimum and maximum frequencies. The number of 
frequency points used with tracking will vary. To illustrate:

H = freq(sys,{Fmin=0.01,Fmax=10,track,delta=.5});

size(H)

ans (a row vector) = 1 

1 

335

The dynamics of this system are adequately reflected in both frequency 
responses. However, systems having more closely-placed pole and zero 
locations are good candidates to use with the 

track

 keyword.

While 

freq( )

 provides you directly with the frequency response, other 

tools in the Control Design module can give you more insight into what the 
open- and closed-loop frequency responses of a system imply about the 
system behavior. Bode plots of system frequency response are useful 

U(s)

Y(s)

(s + 0.5)

s

2

(s + 2)(s + 10)